EP0051018A1 - Method and apparatus for electromagnetic borehole logging - Google Patents

Abstract

Electromagnetic logging tool intended to be moved in a borehole in order to emit electromagnetic energy in the surrounding medium and to detect a fraction of this energy after propagation in the medium to be studied. In one embodiment, it comprises at least one antenna formed by a first conductive element (124) wound in a spiral around a sleeve made of dielectric material (120) intended to be mounted along the probe mandrel. The underside of the sleeve is coated with a second metallic conductive element (126) to which the end (128) of the first conductor is connected by a short circuit (129). The antenna is connected at a point (132) spaced from its end (128) to the core of a coaxial conductor whose sheath is joined to the second conductor (126) for the electrical connection of this antenna with circuits placed at inside the probe. Two-plate type antennas are also provided to be mounted on skids. Application to the production of tools for investigating the electrical permittivity and / or the electrical conductivity of the environments surrounding a borehole.

Description

The present invention relates to the investigation of underground environments using electromagnetic waves.

Techniques for investigating an underground medium traversed by a borehole have long been known, in which a probe is made to circulate in this hole and, depending on the depth of this probe, certain physical characteristics of its medium are noted. surrounding area to obtain logs from which it is possible to obtain useful information for the exploration and / or exploitation of mineral matter from the formations interested in this drilling.

Such measurements call upon various stimulation techniques, including in particular the emission of electromagnetic waves in order to determine certain parameters linked to the behavior of these electromagnetic waves in the medium concerned.

To this latter category belong the measurements of electrical conductivity of formations crossed by a borehole by electromagnetic induction. Embodiments of methods and tools for induction logging are, for example, described in US Patent No. 2,582,314 filed by HC DOLL. A transmitter coil mounted on a probe is excited by an oscillator at a frequency of the order of 20 kHz, for example, to induce currents in the surrounding geological formation. The importance of these currents depends on the conductivity of the formations in which they originate. They circulate along substantially circular lines centered on the axis of the borehole and themselves cause the appearance of an electromotive force in one or more receiving coils mounted on the logging probe at determined distances from the transmitting coil. The analysis of parameters of the output signal of these receiving coils with respect to the signal emitted allows information to be obtained on the conductivity of the formations crossed by these currents.

These conductivity measurements by induction constitute one of the basic techniques for the investigation of geological formations crossed by a borehole. They complement the methods of measuring electrical resistivities using electrode tools. They are particularly essential when the internal environment of the borehole which in exploration drilling, is filled in the general case with a mud intended to stabilize its wall, is not very conductive of electricity and does not allow employment of electrode tools.

More recently, tools have been proposed for measuring certain characteristics of environments surrounding a borehole which involve the propagation of electromagnetic energy in these environments at frequencies significantly higher than the frequency used to carry out induction logging. In these techniques, radio frequencies are used in a range which can range from a low frequency of about 1 megahertz to about one gigahertz.

It has long been known that the characteristic parameters of the propagation of an electromagnetic wave in an environment such as geological formations depend both on the conductivity of these formations and on their dielectric constant.

• dans laquelle e est le symbole de l'exponentielle;
• j est l'opérateur des nombres complexes;
• D est la distance parcourue par l'énergie; et k est une constante de propagation définie par la formule :
  

The attenuation of an electromagnetic wave propagating over a distance D in a medium which tends to dissipate electromagnetic energy varies as the expression:

• in which e is the symbol of the exponential;
• j is the operator of complex numbers;
• D is the distance traveled by the energy; and k is a propagation constant defined by the formula:
  

• est la pulsation pour la fréquence f considérée (ω=2πf) ; µo est la perméabilité magnétique du milieu considéré;
• α est la conductivité du milieu intéressé par la propagation;
• ε est la constante diélectrique ou permitivité électrique de ce milieu.

In this equation:

• is the pulsation for the frequency f considered (ω = 2πf); µo is the magnetic permeability of the medium considered;
• α is the conductivity of the medium interested in the propagation;
• ε is the dielectric constant or electrical permitivity of this medium.

If we consider a non-conducting medium in which equals 0, it follows from expression (II) that the constant k is a real term. The exponent of the exponential of expression (I) is then a pure imaginary term which corresponds only to a phase shift in the expression of the attenuation of the transmitted signals. In other words, the propagation of the waves in this medium is carried out without overall attenuation of energy, only remaining the geometric attenuation of amplitude.

If the conductivity increases (conductive drilling mud by example), there comes a time when the term α becomes much greater than the term j □ ε. Formula (II) then shows that the term k ² tends to become purely imaginary. In expression (I), the exponent then becomes a term having an imaginary component and a real component substantially equal to each other. When the propagation constant k continues to increase with conductivity, the real component of the attenuation increases exponentially with k.

Thus, a very summary analysis allows us to say that as a first approximation the phase shift increases with the electrical permittivity while the amplitude attenuation increases with the conductivity.

In order to measure these parameters characteristic of the propagation of an electromagnetic wave transmitted to a radiofrequency in the formation by a transmitter, use is generally made of at least one pair of receivers spaced longitudinally from one another. The distance from the transmitter closest to these receivers longitudinally in the direction of the borehole determines the transverse depth of the formation that can be reached for the measurement. The distance between the receivers determines the thickness of formation concerned for the measurements of characteristics of propagation of the transmitted wave. These characteristics are, in particular, the relative attenuation of the signals picked up by the receiver closest to the transmitter and by the most distant receiver and the phase shift between the signals received by such a pair of receivers.

The influence of the conductivity of the formations on the attenuation and the phase shift becomes preponderant when the frequency of investigation decreases. On the contrary, as the frequency increases towards the ultra high frequency range, it is the influence of the dielectric constant or electrical permittivity of the formation zone t _I a-paid which becomes preponderant compared to that conductivity.

To obtain measurements of one or other of these characteristic parameters of the formation, namely its electrical conductivity and its electrical permittivity, or dielectric constant, one is led to carry out two simultaneous or consecutive measurements of the propagation of electromagnetic waves for each training area concerned, for example, a relative attenuation measurement and a relative phase measurement.

Thus, it is already known, from United States Patent 4,052,662 issued on October 4, 1977 in the name of RAU, a tool operating in the field of ultra-high frequencies (microwaves) for determining propagation characteristics of electromagnetic waves in the medium immediately surrounding the wall of a borehole crossing formations. This tool includes a probe equipped with pads intended to be applied against the wall of the hole in which it is moved. On this shoe are mounted a transmitting antenna and several receiving antennas of the resonant cavity type. At an operating frequency of 1.1 gigahertz, the attenuation and phase shift of waves picked up by the receiving antennas are measured to derive the value of the dielectric constant of a thin area around the borehole, immediately behind the mudcake. At such high frequencies, the value of the dielectric constant of the medium concerned has a decisive influence on the attenuation and phase shift measurements at the expense of the conductivity, the influence of which on the measurements is increasingly weaker as as the frequency rises. The combination of attenuation and phase shift measures makes it possible to completely eliminate the influence of this last factor to determine the dielectric constant or electrical permittivity of the media concerned.

When one wishes to obtain greater depths of investigation, one is led to space the transmitter and the receivers at distances which make their mounting on skids difficult to practice and one then prefers to install the receivers and transmitter directly on the chuck of the logging probe.

Knowing that the distance to be traveled by waves between the transmitter and the receivers increases with the depth of investigation and that the attenuation of an electromagnetic wave in a medium where it propagates is an increasing function of the frequency, we are then brought to use operating frequencies more Cups, for example 20 to 30 M _H z.

There are thus known electromagnetic logging tools provided with transmitter and receivers mounted on a mandrel at distances which can be of the order of a meter or more to obtain measurements of interest in areas located at a radial distance greater than a meter from to the drilling axis.

Such a tool is described in United States Patent 4,185,238 of January 22, 1980 (HUCHITAL and TABANOU). It includes: a transmitter at the bottom of its own mandrel to operate at a determined frequency; a first pair of longitudinally spaced receivers, the midpoint of which is at a first distance from the transmitter for carrying out a measurement of relative attenuation of the signals coming from the transmitter through the surrounding medium; and a second pair of longitudinally spaced receivers, the midpoint of which is at a second distance from the transmitter, greater than the first distance, for carrying out a measurement of phase shift or of relative phase between the signals which reach them. The first and second distances are selected such that the attenuation and phase shift measurements made by the first and second pairs of receivers respectively concern the same area of the formation. It has in fact been determined that the relative attenuation and phase measurements of waves propagating through the formations were affected in a different manner by the distance between the zone concerned and the axis of the hole. Thus, to obtain measurements of the phase shift caused by the propagation of the waves in a training area tion at a determined distance from the borehole, a pair of receivers must be used located at a greater distance from the transmitter than that which separates it from the pair of receivers intended for wave attenuation measurements in this same area.

In general, the radiation transducers, transmitters or receivers, used for electromagnetic propagation logging must meet certain conditions. In particular, they must be adapted to the transmission of energy in highly dissipative media, that is to say where it is accompanied by considerable losses. In emission, these transducers must therefore be capable of transmitting significant amounts of energy to the surrounding environment; on reception, they must on the contrary be capable of picking up signal levels which are sometimes extremely weak. Furthermore, these transducers must have particular characteristics of directivity. In general, research is carried out in electromagnetic logging techniques to promote the propagation of waves in the direction of formations at the expense of the propagation of these waves longitudinally in the borehole. It is therefore important to provide that the transducers used for this purpose have well-defined directivity characteristics.

According to a technique previously proposed with regard to the choice of transducers, for example by the English patent? 1 088 824, each receiver forms with the transmitter the frames of a capacitor, the dielectric of which is formed by the surrounding medium. To obtain a sufficient investigation depth, these transducers are mounted like antennas on the mandrel and the investigation frequency used is located at the bottom of the radio frequency range. However, this technique does not allow satisfactory results to be obtained, in particular because the antennas are short-circuited by the drilling fluid when the latter is, as is frequent, a relatively conductive medium. In these conditions, it is not possible to obtain nir an efficient transmission of electromagnetic energy in the formation that one seeks to study.

According to another previously proposed technique (see for example the USSR author's certificate in the name of DAEV No. 177 558), the transmitters and receivers used for the transmission of electromagnetic waves between the tool and the surrounding medium are toroidal coils whose axis is directed along the axis of the mandrel of the drilling tool.

In the realizations of Patents USA 3,891,916 of June 24, 1975 (MEADOR and AL.) And 4,185,238 of January 22, 1980 (HUCHITAL and TABANOU), coils are also used for the emission and reception of electromagnetic waves . These coils work like dipoles which have good directivity.

The use of coils makes it possible to remedy to a certain extent the faults reported about the antennas forming capacitor plates. In particular, these coils are capable of operating in slightly conductive drilling fluids. However, it has been found that this improvement has limits and that the level of the signals which reach the receivers after propagation through the formations concerned is often extremely low.

Considerable precautions are necessary to avoid deterioration of the signals received by the receivers for further processing. We are also led to provide extremely sensitive electronic measurement circuits which make the production of the tool more difficult and its operation more complex. This is particularly so for the circuits associated with a pair of coils relatively distant from the transmitter such as, for example, in the embodiment mentioned above with reference to United States Patent 4,185,238 (HUCHITAL and TABANOU), the circuits that are assigned to the measurement of phase shifts corresponding to the training area studied.

In addition, we have seen that the attenuation of an electromagnetic wave in a medium in which it propagates increases strongly with the electrical conductivity of this medium. Thus, with known tools, when the resistivity of the drilling mud becomes less than 0.1 ohm per meter, the attenuation of the waves in the drilling mud during transmission and reception is such that it does not it is no longer possible to obtain exploitable indications from the receivers on the propagation of waves through the formations considered.

Attempts have been made to increase the power radiated by the coils by increasing their diameter, which also has the advantage of reducing the thickness of drilling fluid to be traversed by the electromagnetic energy leaving or arriving at these coils and l 'corresponding attenuation. However, the consecutive increase in the external diameter of the tool is limited by the size of the holes in which the tool is intended to be used and by considerations of space.

In the field of aerial and space telecommunications, it is known to use antennas constituted by a dielectric plate on one face of which is printed an elongated conductive element while its other face is metallized to form a second conductive element or plane of mass. These antennas known as two-plate lines use techniques for manufacturing printed circuits. They have the advantage of being able to be relatively easily shaped to the shape of air or space vehicles in a relatively small volume. They have proven to be well suited to omnidirectional type transmissions in air or vacuum where the propagation of electromagnetic waves is practically carried out without loss. The performance of antennas of this nature, which increases as the square of their frequency of use, is acceptable in the application to telecommunications at frequencies of several hundred megahertz. It is not as good as that of conventional aerial antennas which are sized in function tion of the wavelength of propagation of the radiations which they are called to transmit in air or vacuum. However, taking into account the weak attenuation exerted by the propagation medium on these radiations, the loss of efficiency which accompanies the use of bi-plate antennas in the application considered is compensated by their advantages, particularly in terms of size.

In what follows, the terms "transmission" and "transmit" will be used in relation to the propagation of a radiation in a medium indifferently with regard to the emission and the reception of this radiation by a transducer immersed in this medium.

One of the objects of the present invention is to improve the performance of electromagnetic logging tools, in particular by making it possible to increase the power of the radiation transmitted by these tools for measurement purposes under the conditions of use prevailing in the boreholes.

According to one aspect of this invention, a logging tool suitable for being moved in a borehole crossing geological formations for measuring the propagation characteristics of an electromagnetic wave in the surrounding medium, comprises a probe and means associated with this. probe: for emitting electromagnetic energy towards the surrounding medium and for detecting part of this energy after its propagation in said medium, and, is characterized in that said means comprise at least one antenna comprising a first elongated conductor of electricity and a second conductive element disposed opposite and at a distance from the first element over the entire useful length of the latter, the opposite portions of the first and second elements being separated by a non-conductive medium and electrically connected to each other at one end of the useful length of the first element. According to another aspect, the invention relates to the logging method in which such an antenna is implemented.

Thus, a radiation transducer is used real antenna that can preferably be tuned depending on the frequency of the radiation to be transmitted. To this end, according to one embodiment, the electrical connection between the two elements is a short circuit and the useful length of the first element is granted as a function of the wavelength of propagation of said electromagnetic energy in the non-conducting medium. .

It has been found that the antennas which have just been defined could be advantageously adapted for use in logging tools for emitting or receiving radiation in the environment of a borehole with performance in terms of much higher yield. to those which were accessible with the radiating systems of the prior art in this field.

Such antennas can be produced in the so-called bi-plate line technology according to which the first elongated conductive element is printed on one face of a plate of insulating material, while the other face is metallized to form the second conductive element or ground plan. As indicated above, this technology has been used to form antennas suitable for the omnidirectional transmission of electromagnetic radiation in non-dissipative media such as air or vacuum, that is to say in which the propagation of these waves are practically lossless, which is essential for the transmission of information.

However, it has been realized that it is possible to operate these antennas with good performance in very highly dissipative media, such as the media concerned with logging measurements in boreholes to carry out measurements of the degradation of the electromagnetic signals transmitted. It has also been found that in environments surrounding the boreholes, and in particular in liquids, such as the drilling muds in which they are immersed, these antennas functioned, with very good efficiency, at frequencies of a few tens of megahertz bets used in deep investigation electromagnetic logging tools. It is notably remarkable, as will be explained in more detail below, that the yields obtained are much higher than those which the same antennas would provide in air or in vacuum for the same frequencies. Such an effect is of course very favorable to applications to measurements relating to the propagation of radiation in such media. In fact, the more the radiated power in transmission and the signal power to the receiver after propagation in the medium to be studied are available, the easier and more precise the attenuation and / or phase shift measurements can be.

It has further been observed that it is possible to conform antennas of the type defined above so as to give them characteristics of directionality suitable for logging in boreholes. Thus, by adjusting the mechanical length of the antenna mounted for example on the mandrel of a logging tool, it is possible to cause a phase shift of the currents along this antenna suitable for favoring the propagation of the radiation in the ambient medium in directions transverse to this mandrel between a transmitting antenna and a receiving antenna.

Finally, the application of the antennas which have just been defined to logging tools in drilling is accompanied by a certain number of specific beneficial effects of this type of application which will be reviewed later in more detail.

According to one embodiment, the non-conductive medium is a plate of dielectric material. According to another embodiment particularly suitable for applications of the tool at low frequency operations, the non-conductive material is a magnetic material with high magnetic permeability.

According to a preferred embodiment, the impedance of each antenna is adapted to the impedance of a coaxial link which connects it to electronic circuits by adjusting the position of the connection point of the core of this coaxial link along the first conductive element.

According to one embodiment, the antennas are mounted on tool chucks. It is therefore particularly advantageous to have the first conductive element around this mandrel to reduce its mechanical length parallel to the longitudinal dimension of the latter, while the second element extends over two dimensions by forming a cylindrical mass element disposed at the inside the first. This second element can then act as a screen vis-à-vis the first against the disturbing influences of the signals inside the probe, for example of the supply current of an oscillator supplying the transmitter. It is possible, in particular, to extend this second element beyond the antenna, so as to form a continuous screen tube along the probe inside which the supply conductors of the tool circuits pass. . In addition, various embodiments are then provided to improve the protection of the antenna, while minimizing or eliminating the propagation of electromagnetic energy in transverse mode (TEM mode) from the transmitter to the receivers and to obtain a better arrangement of the internal space of the tool.

According to a particularly advantageous application of the antennas according to the invention, in the case where the second element of the antenna forms a cylindrical element of mass inside the first element, it is possible to extend the latter on both sides and d 'other of the antenna to put it in direct electrical contact with 1 outside of the probe. The probe is then advantageously formed by a tubular steel body, a portion of which is used to form the second element of the antenna, which is covered with a dielectric material which supports the first element or radiating element of the antenna.

This leads to a new design of an electromagnetic logging tool in which the probe body is conductive. This design can find applications in all areas of frequency of use of the tools, including at the lowest frequencies. It also makes it possible to envisage the combination of tools implementing electromagnetic radiation with tools using other types of physical phenomena. According to one embodiment, a metal electrode for current conduction through the formation is equipped for resistivity measurements with an antenna of the type indicated above in which the second conductive element is electrically linked with the body of the electrode.

In general, one can expect a quite considerable increase in efficiency of the antennas fitted to the tools according to the invention compared to the coil antennas traditionally used in drilling logging tools. The powers available for measuring the propagation of radiation are of an order of magnitude much higher than that which was accessible before. This improvement has remarkable consequences in terms of the performance of the tools according to the invention as well as their field of application, in particular when these antennas are used both as receivers and as transmitters, the efficiency of their system. radiant being in fact multiplied by several thousand times.

In particular, the tools provided with antennas of the indicated type allow an investigation of the environments surrounding a borehole in which the resistivity of the drilling mud is significantly lower than that which was considered until now as minimal for the correct operation of tools for electromagnetic propagation. They also have various advantages in terms of the physical production of such tools, in particular their design, both exterior and interior.

The antennas defined above apply particularly well to the mounting on mandrels of deep investigation tools which are intended to operate at frequencies of a few tens of megahertz, like the tools with coils described above.

It has also been found that these antennas could advantageously also be applied to electromagnetic logging tools in the relatively low frequency range, below 10 megahertz for example, especially when a material with high magnetic permeability is used between the two conductive elements of the antenna, as well as in the higher frequency range, above 200 megahertz for example. In particular, in the latter case, antennas suitable for being mounted on pads are produced in a simple manner.

• la figure 1 représente schématiquement un outil de diagraphie électromagnétique auquel est appliquée l'invention;
• la figure 2 est un schéma électrique équivalent au branchement d'une antenne de l'art antérieur;
• la figure 3 est une représentation schématique en perspective d'une antenne du type biplaque;
• la figure 4 est un schéma de constitution et de branchement d'une antenne biplaque;
• la figure 5 est un schéma équivalent au schéma de la figure 4;
• la figure 6 est une représentation d'une antenne pour mandrin conformément à l'invention;
• la figure 7 représente une autre forme de réalisation d'une antenne pour mandrin;
• la figure 8 représente un schéma de montage d'une antenne du type de la figure 7 sur un outil de diagraphie;
• la figure 9 représente une variante de réalisation d'une antenne pour mandrin;
• la figure 10 représente une sonde de diagraphie équipée d'antennes sur mandrin conformes à l'invention;
• la figure lla représente une variante de réalisation d'un outil semblable à ce ui de la figure 10;
• la figure llb est unt vue agrandie d'un détail de la figure lla coupé par un plan diamétral longitudinal;
• les figures 12 et 13 sont des schémas de branchement d'antennes biplaques dans certains cas d'utilisation;
• la figure 14 représente une antenne biplaque en forme de bouton;
• la figure 15 représente un patin pour outil de diagraphie comportant des antennes biplaques;
• la figure 16 représente un autre mode de réalisation d'un patin conformément à l'invention;
• la figure 17 montre encore une autre forme de réalisation d'un patin conforme à l'invention;
• la figure 18 représente schématiquement la constitution d'un outil combiné à électrodes et à antennes conforme à l'invention.
• Un forage 20 (figure 1) limité par une paroi 28, traverse des formations géologiques 22 depuis la surface du sol 24 dans une direction sensiblement verticale dans cet exemple. Il est rempli d'un fluide de forage 26, la densité de ce fluide ou boue de forage étant déterminée et ajustée pour que la pression hydrostatique exercée par ce fluide sur la paroi 28 du forage 20 équilibre la pression interne des formations traversées et permette ainsi de maintenir l'intégrité de la paroi 28. Dans le forage 20 est suspendu un outil de diagraphie électromagnétique 30 à l'extrémité d'un câble 32 propre à le supporter mécaniquement lors de ses déplacements dans le forage 20 ainsi qu'à permettre de relier électriquement l'outil 30 à une station de surface 34. Le câble 32 passe, à cette station, le long d'un galet 36 dont le déplacement angulaire permet de suivre les variations de profondeur de l'outil et de commander l'entraînement d'un support d'enregistrement, magnétique ou photographique par exemple, dans un enregistreur 38 pour effectuer un relevé ou diagraphie de données transmises par l'outil 30 à travers le câble 32, en fonction de la profondeur de cet outil 30.

Other aspects and advantages of the invention will appear on reading the following description given by way of example, with reference to the appended drawings, in which:

• FIG. 1 schematically represents an electromagnetic logging tool to which the invention is applied;
• Figure 2 is an electrical diagram equivalent to the connection of an antenna of the prior art;
• Figure 3 is a schematic perspective representation of an antenna of the twin plate type;
• Figure 4 is a diagram of constitution and connection of a two-plate antenna;
• Figure 5 is a diagram equivalent to the diagram of Figure 4;
• Figure 6 is a representation of a mandrel antenna according to the invention;
• Figure 7 shows another embodiment of a mandrel antenna;
• FIG. 8 represents a diagram of assembly of an antenna of the type of FIG. 7 on a logging tool;
• FIG. 9 represents an alternative embodiment of an antenna for a mandrel;
• FIG. 10 represents a logging probe equipped with antennas on a mandrel according to the invention;
• Figure 11a shows an alternative embodiment of a tool similar to that of Figure 10;
• Figure 11b is an enlarged view of a detail of Figure 11a cut by a longitudinal diametral plane;
• Figures 12 and 13 are diagrams of connection of two-plate antennas in certain use cases;
• FIG. 14 represents a button-shaped bi-plate antenna;
• FIG. 15 represents a pad for a logging tool comprising two-plate antennas;
• FIG. 16 shows another embodiment of a skate according to the invention;
• FIG. 17 shows yet another embodiment of a shoe according to the invention;
• FIG. 18 schematically represents the constitution of a tool combined with electrodes and antennas according to the invention.
• A borehole 20 (FIG. 1) limited by a wall 28, crosses geological formations 22 from the surface of the ground 24 in a substantially vertical direction in this example. It is filled with a drilling fluid 26, the density of this drilling fluid or mud being determined and adjusted so that the hydrostatic pressure exerted by this fluid on the wall 28 of the drilling 20 balances the internal pressure of the formations crossed and thus allows to maintain the integrity of the wall 28. In the borehole 20 is suspended an electromagnetic logging tool 30 at the end of a cable 32 adapted to mechanically support it during its movements in the borehole 20 as well as to allow electrically connect the tool 30 to a surface station 34. The cable 32 passes, at this station, along a roller 36 whose angular displacement makes it possible to follow the variations in depth of the tool and to control the drive of a recording medium, magnetic or photographic for example, in a recorder 38 to carry out a reading or logging of data transmitted by the tool 30 through the cable 32, as a function of the depth of this tool 30.

The tool 30 comprises an elongated probe body or mandrel 40 suspended by its upper end 42 to the cable 32 and which comprises an outer casing 44 suitable for isolating the functional parts of the drilling tool 20. In the vicinity of the inner end from the mandrel 40 is mounted a transmitter 50 (T) constituted by an antenna suitable for the emission of electromagnetic energy at radio frequencies in the immediate environment of the borehole 20 and the neighboring formations 22. Above the transmitter 50 is mounted, on the mandrel 40, a first pair of receiving antennas 51 and 52 (R1 and R2) vertically spaced at a predetermined distance. The distance between the emitter 50 (T) and the medium L'2 of the interval separating the receptors R1 and R2 is marked by D _n .

Above this pair of antennas is mounted, on the mandrel 40, another pair of spaced apart longitudinal receiving antennas 53 and 54 (R3 and R4). The middle L ' ₃ of the interval between these antennas is located at a distance D _f from the transmitter 50 greater than D _n .

The transmitter 50 is capable of emitting an electromagnetic wave in a dihedral of 360 ° around the axis of the mandrel 40. It is supplied from an oscillator 60 housed inside the envelope 44 via a coaxial link 62. The oscillator 60 also controls an oscillator 64 capable of operating at a slightly higher or slightly lower frequency (a few tens of kilo hertz apart).

The receivers 51, 52 (R1 and R2) are capable of detecting electromagnetic radiation which reaches them after having propagated through the formations 22 in a dihedral of 360 ° around the axis of the borehole 20. They are connected to an amplitude comparator 66 receiving the output frequency of the oscillator 64 on an input 67 ..

The receivers 53 and 54 (R3 and R4) are connected to the inputs of a relative phase detector 68 which receives on an input 69 the output frequency of the oscillator 64.

The receiving antennas 51, 52 are connected to the amplitude comparator 66 by coaxial cables 56 and 58 respectively on the receiving antennas 53 and 54 are connected to the phase detector 68 by coaxial cables 55 and 57 respectively. The two circuits 66 and 68 each include a mixer of signals from the oscillator 64 and signals received from the antennas 51, 52 and 53, 54 respectively, in order to derive therefrom a signal whose frequency is relatively low (a few tens of kilo hertz), to determine on the one hand the difference in amplitude of the signals received by the receivers R1 and R2 and, on the other hand, their phase shift from the signals received by the receivers R3 and R4. The two corresponding pieces of information are available at the outputs 70 and 71 of the circuits 66 and 68, respectively and are transmitted to the surface by the cable 32 to a processing member 72 suitable for supplying the recorder 38 with logging signals representing by example the dielectric constant and / or the conductivity of the formations interested in the propagation of the waves emitted by the transmitter 50. The cable 32 also ensures the power supply of the oscillators 60 and 64, as well as of the electronic circuits 66 and 68 which are housed in the tool or in the probe 40.

Tools having the general structure which has just been described with reference to FIG. 1 are known in the prior art, for example by the aforementioned United States Patent 4,185,238 (Huchital and TABANOU).

However, it has been found that the use of these tools comes up against a certain number of limits which are due to the insufficiency of the powers of electromagnetic radiation which they make it possible to bring into play in the formations to be studied. Thus, for example, knowing that the attenuation of the electromagnetic waves which propagate in a medium increases with the conductivity of this medium, it is not possible to use or envisage the use of such tools in sludge of drilling with a resistivity of less than 0.1 ohm per meter, which significantly reduces the field of application.

In these tools, the transmitters 50 and the receivers 51 and 52 are generally constituted by coils comprising a small number of turns, for example two, mounted on an insulating sleeve such as a ceramic. We have tried to increase the radiation power transmitted using these coils both in transmission and in reception. However, it has been found that these efforts come up against limits in the dimensioning of drilling tools and in the electrical powers which can be used in a tool connected to the end of a cable several thousand meters long. .

An antenna 99 of the two-plate type (FIG. 3) is formed by the combination of two metallic elements plated on either side of a dielectric according to a technique which is similar to the manufacture of printed circuits. A plac e to ^p planar dielectric 100 comprises on one of its faces 101 a conductive strip 102 made of copper, of curvilinear shape in this example, printed in the dielectric 100. This dielectric may for example be a high temperature ceramic. The opposite face 103 to the face 101 of the plate 100 is entirely covered with a metallic coating 105, for example made of copper, aluminum or invar. One end 106 of the metal strip 102 is electrically connected to the metal sheet 105 by a short-circuit link 108 through the dielectric 100. A coaxial cable 109 reaches the antenna 99 from the side of its underside 103. The sheath of this coaxial cable l09 is electrically connected to the plate 105 while its core, after having passed through the dielectric, is welded to the blade l02 at a point 110 placed at a predetermined distance from the end 106. The blade l02 has a length curvilinear on the plate 100 between its end 106 and its other end left electrically free (represented at 107 in FIG. 4), equal in this example to a quarter of the wavelength of propagation of the operating frequency of l 'antenna.

The sheet 105 is generally designated by the name of the trap plane, the strip 102 being considered as forming the radiating element proper.

In general, we consider here antennas comprising a first and a second metal elements facing each other such as the strip 102 and the sheet 105, separated by a non-conducting medium constituted, for example, by a solid plate of a material dielectric 100. These elements are electrically connected at one of their ends, the length of the first element being determined as a function of the wavelength with which the electromagnetic signals propagate in the dielectric separating the two conductive elements at the frequency considered. We know that when the length of an antenna transmitting electromagnetic radiation is equal to a quarter of the wavelength of this radiation, or to a multiple of the latter, the input impedance of this antenna is purely real, which is a condition for optimal performance of this antenna.

In this type of antenna, the conductive elements separated by the dielectric, are arranged opposite; on the other hand, it is not essential for their operation that the second conductive element, corresponding in the example of FIG. 3 to the plate 105, extends over a surface. It is possible to produce antennas in which the second conductor is also an elongated element following a path substantially parallel to that of the first conductor and arranged opposite the latter. The arrangement that this second element extends in two dimensions along a plane or a mass surface has a number of advantages, some of which will be discussed below.

When the antenna operating frequency is relatively low, for example of the order of a few tens of megahertz, a quarter of the corresponding wavelength in air represents a relatively large length compared to the dimensions of the tools. well logging. In the case of a frequency of 25 megahertz, a quarter of the wavelength of the propagation of radiation in the air is equal to about 3 meters. When the antenna, instead of being mounted in air, is placed in a dielectric material (Figure 3 for example), the wavelength radiated in this dielectric for a determined frequency is less than that of the radiation in the due to the higher value of the dielectric constant of the material.

This can be explained by considering the relation (II) defining the propagation constant. When the conductivity has of the medium in which the antenna is placed is zero, one can write the relation:

on peut écrire :

Knowing moreover that when the conductivity is zero, the propagation constant is

we can write :

It is verified that if the electrical permittivity ε of the dielectric material on which the first element is printed is greater than 1, that is to say the permittivity of air or vacuum, the propagation wavelength of the radiation in this medium decreases as a function of the square root of this permittivity. For a dielectric medium with a permittivity equal to 4, the corresponding wavelength will therefore be approximately half that of propagation of the radiation at the same frequency in air. It follows that the length of an antenna tuned to a quarter or a half wavelength can be halved when this antenna is placed in a dielectric. Under these conditions, at 25 megahertz, the antenna length necessary to obtain a good transmission efficiency in this dielectric is equal to 1.5 meters. Various arrangements are provided for making these antennas in a minimal space requirement on a drilling logging tool.

When the frequency of the signals to be transmitted increases, the corresponding wavelength decreases and it is then possible to consider giving the antenna lengths equal to half a wavelength of the radiation in the dielectric. In the latter case, the two ends of the first element such as l02 are short-circuited with the ground plane 105. Of course, as the frequency of the electromagnetic waves to be transmitted increases, it is possible to increase the length of the antenna relative to the wavelength in the dielectric, for a given bulk, which is, in all cases, a factor for improving the efficiency.

The tests carried out with antennas of the type which comes to be defined have shown on the one hand that these antennas could be applied satisfactorily to the exchange of radiation energy with strongly dissipative media for the study of the propagation of electromagnetic radiation in these media and, on the other hand on the other hand, that they made it possible to achieve quite considerable yields, both in transmission and in reception, for the measurements of electromagnetic propagation in boreholes compared to the coils traditionally used in this field.

With regard to the first observation, it should be noted that the properties of the two-plate lines have been discovered and used for the transmission of electromagnetic signals in environments where the attenuation due to the propagation of these signals is extremely low. However, tests have shown that not only the antennas of the type which have just been described can function satisfactorily in highly dissipative media but also that, in the particular case of logging tools in boreholes, which are called to function in a drilling mud, the efficiency of these antennas was higher than those in the air or vacuum, all other things being equal.

In the air, these antennas have an efficiency which increases like the square of the transmitted frequency. When used in the context of space or air telecommunications, they involve frequencies of several hundred megahertz with a performance which, although less good than that of conventional aerial antennas, is however acceptable for the needs of communications by transmission. in a non-dissipative medium. However, tests have shown that these antennas could be used with excellent performance in the environment of boreholes at frequencies much lower than their current frequency of use in air or vacuum. This is particularly the case at frequencies of a few tens of megahertz corresponding to the frequencies used in professional tools electromagnetic deep investigation paging.

• dans laquelle R_r est la résistance de rayonnement de l'antenne
• est la longueur mécanique ou hauteur efficace de cette antenne; et
• est la longueur d'onde de propagation du rayonnement dans le milieu dans lequel est plongée l'antenne.

An explanation of this observation can be advanced if we consider the approximate relationship:

• where R _r is the radiation resistance of the antenna
• is the mechanical length or effective height of this antenna; and
• is the wavelength of propagation of the radiation in the medium in which the antenna is immersed.

The energy of the radiation received or emitted by an antenna is of an active nature. We call radiation resistance R _r the value of the fictitious resistance which would produce a heat dissipation of energy equivalent to the radiated energy. Thus, the greater the radiation resistance of an antenna, the greater the power that it is capable of transmitting. The efficiency η of the antenna can be characterized by considering, on the one hand, the radiation resistance corresponding to a useful energy transmission and, on the other hand, a loss resistance R:

dans laquelle λo est la longueur d'onde du rayonnement dans l'air.If we consider the radiation resistance R _ro of an antenna immersed in air or vacuum:

in which λo is the wavelength of the radiation in the air.

In water, the wavelength of the radiation is, conformed ment to relation (V) above:

However, the dielectric constant of water ε _r is equal to approximately 80, which amounts to saying that the resistance R in water is approximately 80 times stronger than in air. It follows that for a determined efficiency n of the antenna in air, the corresponding efficiency of the same antenna in water is very substantially higher and close to 1 insofar as the loss resistors R which have not changed can then be considered negligible compared to the radiation resistance of the antenna in water.

The relation (V) is valid for a nonconductive medium. In practice, the consideration of the relation (II) defining the propagation constant makes it possible to note that if the conductivity c of the surrounding medium in the antenna is not zero, the propagation wavelength tends to decrease further compared to at its value in a non-conductive medium.

It has also been found that antennas of the general type considered previously could effectively be adapted to the specific conditions of their use on logging tools. Thus, these antennas can be shaped as a function of the space constraints of these tools and give them a certain directivity in order to favor transmissions of radiant energy in directions transverse to the direction of drilling.

In practice, the plate of insulating material can be mounted so that one of its faces on which the first conductive element is printed appears on the surface of the tool, either on a mandrel or on a shoe. This conductive element can be wound up if necessary on the surface of this mandrel or of this shoe in order to give the antenna the desired electrical length while maintaining its mechanical length in dimensions compatible with those of the tool. In furthermore, this mechanical length can be adapted as a function of the directivity characteristics provided for the tool. The directivity of an antenna depends on the phase shifts of the currents per unit of mechanical length along this antenna. It is advantageous that the antenna length is sufficient to obtain current phase shifts which favor the action of the antenna in certain directions at the expense of others, which is obtained in particular by providing a mechanical length of the antenna of the order of magnitude of the wavelength of the radiation in the medium external to the antenna.

With regard to the second observation made during the tests reported above with the new type of antenna, we can put forward a number of explanations for the exceptional improvement in performance observed compared to the coils traditionally used for the type of measure considered.

In particular, it has been determined that the efficiency of conventional coils is extremely low due to the combination of a number of factors. We note in particular (FIG. 2) that a radiation transducer such as the emitter 50 formed by a coil 81 can be represented by the equivalent diagram of a self inductance 80 in series with a resistor 82 connected to the output of a coaxial cable 84 supplied by an oscillator 86. The impedance of such a coil 81 is very highly reactive due to the high value of the inductor 80, while the coaxial link 84 intended to supply this coil is designed to transmit a energy to radiate of an essentially active nature. This lack of adaptation of the impedances is in itself the cause of a poor efficiency of the very high frequency energy transmission between the oscillator 86 and the antenna 81. It results in fact in the establishment of a standing wave system between the oscillator 86 and the input of the antenna 81 through the coaxial link 84 whose maintenance absorbs a very large fraction of the energy which passes through this liaison. Under these conditions, the efficiency of the energy transmission between the oscillator 86 and the coil 81 hardly exceeds 10%. The same phenomenon is observed for the coaxial links of the receiving antennas.

Furthermore, it has been found that only part of the energy reaching the coil 81 (in the case of an emitter) is transformed into radiant energy capable of propagating outside the tool 30, this energy corresponding to what has previously been called radiation resistance. There are indeed relatively considerable losses by the Joule effect in the winding of the coil itself. To these ohmic losses are added dielectric losses resulting from the capacitive connection between the conductors of the coil itself and the other parts of the tool which are grounded. Thus, the power radiated outside the coil represents only a fraction of a few percent of the power actually reaching this coil. In total, the power radiated by a radiant coil system is little more than about 1% of the power available at the output of the power oscillator. The same phenomena affect the efficiency of these coils when they are used for reception.

Compared to these coils, the radiation transducers of the type considered in accordance with the invention constitute real antennas, the length of which can be tuned as a function of the frequency of the radiation that they are called upon to transmit so as to give them essentially an impedance. active, condition of a good performance of their operation. Thus, the impedance of these antennas is very weakly reactive, unlike the case of coils; and this result can be achieved thanks to the arrangements indicated above, with dimensions compatible with those of the tools intended to be used in the boreholes.

This essentially active impedance has advantages not only for establishing a condition of resonance favorable to the conversion of electrical energy into electromagnetic energy by the antenna proper, but also in terms of the efficiency of the transmission of electrical energy along the coaxial link connecting the antenna to its associated electronic circuit. Losses if- g _n alées previously in coils are avoided effect.

These antennas also offer the possibility of carrying out, in a very simple manner, an impedance adaptation of the antenna to the impedance of the coaxial cable which connects it to the electronic circuits for supplying or processing the signals.

A diagram of a developed longitudinal section view of a two-plate antenna is given in FIG. 4 in which the same references have been used as in FIG. 3. An oscillator l12 is connected to one end l13 of the coaxial link 109 whose the other end is connected by its sheath 114 to the ground plane 105, perpendicularly to the latter, the core 115 of this connection being isolated from the ground plane 105 and passing through the dielectric 100 to reach the junction point 110. The ground plane 105 constitutes an electrical screen between the blade 102 constituting the sensitive part of the antenna 110 and the electronic circuits located behind the ground plane. The junction point 110 of the core 115 of the coaxial l09 is located at a distance d from the short-circuited end 106 of this antenna 99, which is chosen to carry out an impedance adaptation by making the element play 102 the role of an auto-transformer, as it will be explained.

A diagram equivalent to the antenna 99, represented in FIG. 5, comprises, between the ends 106 and 107 of the strip 102, a wound coil L whose number of turns corresponds to the length of this strip and a capacity C equivalent to the capacitive link existing between the blade 102 and the ground plane 105. The ground plane 105 is connected to ground. The LC circuit in parallel represents a resonant circuit equivalent to the antenna. For an antenna length equal to a quarter of the wavelength radiated in the dielectric, the impedances of the circuit containing the self L and of the circuit containing the capacitance C correspond to the resonance condition which allows the optimal conversion of electrical energy into radiant energy. A resistor R connected in parallel to the components L and C, between the ends 106 and 107 of the antenna, represents an impedance equivalent to the losses by conduction in the antenna and to the radiated power (radiation resistance), this power being, as previously indicated, a purely active power. The junction point 110 is an intermediate point of the choke which corresponds to a number of turns ni of this choke between the ground plane 105 and the junction 110, n _t being the total number of turns of the choke L. The position of the junction 110 is chosen so that the quantity (n _i / n _t ) ² x R is substantially equal to the impedance of the coaxial connecting line 109. The self L plays the role of a self-transformer whose can calculate the impedance reported at the input, _i.e. (n _i / n _t ) ² x R. If this impedance is equal to the impedance of the coaxial line used with the antenna, energy losses are completely eliminated by reflection in the coaxial, which contributes to obtaining a good efficiency of the antenna.

The potential increase in the power available in the environment specific to logging tools in boreholes which results from the use of biplate type antennas has very important consequences for the applications and the realization of such logging tools. electromagnetic logging. In addition, certain technological features of bi-plate antennas have other advantageous consequences which will be examined below in connection with various embodiments and use of antennas for electromagnetic logging tools.

For example, in a bi-plate antenna for a mandrel, FIG. 6, a first conductive element 124 of the antenna is wound in the form of a helix of regular pitch around a cylindrical sleeve 120, of dielectric material, the internal face of which is upholstered by a conductive sheet 126 forming the second conductive element or ground element. The conductive element 124 is a copper strip printed in the surface 122 of the sleeve 120 over a developed length equal to a quarter of the wavelength of propagation of the radiation at the frequency considered in the material constituting this sleeve 120. The pitch of the winding of the blade 124 is relatively tight and the longitudinal spacing between the turns of this blade is substantially less, as shown, than the width of this blade 124.

The sleeve is made of a ceramic or a composite material based on fiberglass and resin. The blade 124 is implanted there according to a conventional printing technique of the type used in the production of a printed circuit, and for example by etching or electrodeposition. The lower end 128 of the first element 124 is electrically connected to a power source at high frequency by a coaxial link 130 having a core connected at a point 132 of the internal face of the helical blade 124 (that is to say its face in contact with the dielectric 120). The core of this coaxial connection crosses this dielectric perpendicular to the internal surface of the dielectric sleeve 120, the sheath of the coaxial being connected to the cylindrical mass element 126.

Such an antenna is suitable for being mounted coaxially with the mandrel of a drilling tool such as that shown in FIG. 1 to constitute the transmission 50 and reception 51 and 52 elements. For example, an antenna produced at this effect comprises eight turns of a metal blade one-tenth of a millimeter thick of copper whose width (dimension measured in the axial direction) is 5 mm, the pitch of the propeller being 7.5 millimeters, on a sleeve with an external diameter of 8 cm.

The internal surface of the sleeve is lined with a metal sheet one millimeter thick forming a ground plane. The insulating sleeve consists of a polysulfone and has a thickness of 5 millimeters. For an operation planned at 25 megahertz, the antenna tuned to the quarter of the wavelength of the wave of propagation of this frequency in the dielectric material of the sleeve 120 has a developed length of approximately 2 meters and a total length in the axial direction or mechanical length of 6 centimeters. Measurements of the yield obtained with such an antenna operating in salt water provided values of the order of 90%, that is to say of an order of magnitude incommensurate with what was conceivable with the front coils.

The length of the antenna being connected to a quarter of the wavelength of the radiation in the dielectric, the resonance condition of this antenna is met without any other particular arrangement and in particular without the need to provide circuits for special agreements. Such circuits were necessary with coil transducers to allow the circuit of the latter in the resonance condition. The elimination of these tuning circuits is an advantage because they are difficult to manufacture and subject to radiant energy leaks and they occupy a non-negligible space in the tool.

The transmitting antenna and the receiving antennas are separated by portions of insulating tubes to which they are connected by suitable means, for example by connection sleeves. According to another arrangement, these antennas are arranged around a common internal insulating sleeve supported longitudinally in the mandrel 40. According to a variant, the casing 44 of the probe body is formed, at least over part of its length, by a extension of an antenna sleeve such as 120 (FIG. 6) over the useful height of the mandrel, the same sleeve then serving as a support for the first and second conductive elements of the various transmitting and receiving antennas.

According to one embodiment, the mass elements such as 126, which constitute the second element of each of the antennas _T , _Rl , R2 (Figure 1) are part of a single conductive cylindrical sleeve extending over the entire height. This arrangement has the advantage that the conductive sleeve thus formed inside the tool acts as a screen with respect to the transmitting and receiving antennas vis-à-vis all disturbing signals circulating in conductors inside the body of the tool, such as, for example, the supply current of the oscillator 60 which generates the power to be emitted in the formations.

In this regard, the electromagnetic propagation tools include electronics for processing the signals picked up by the receiving antennas R1 to R4 (Figure 1) which must be located, in particular because of the low level of these signals near these receivers. This electronics is normally housed either at the upper part of the probe body ₄₀ , or in a specially fitted box above this probe body to which the cable 32 is attached. The transmitter T must therefore be located below the receiver towards the lower part of the tool 30 and its power oscillator 60 must be placed nearby in order to limit the length of the coaxial link 62 which supplies it. The supply of this oscillator by electrical conductors coming from the surface through the cable 32 requires the passage of the probe body by these conductors which therefore pass close to the receivers Rl, R2, etc ... Given the low level signals circulating in the circuits of these = receivers in response to the radiation received from the surrounding environment, it is necessary to remove these circuits from the disturbing influence of the currents induced by the power supply conductors of the oscillator 60.

A solution envisaged and applied in certain cases to avoid this problem is to supply this oscillator 60 with a battery during the measurement periods when the transmitter transmits radiation towards the formations, this battery then being suitable for being recharged outside the periods of operation of electromagnetic detection. However, this solution requires the use of a battery in the difficult environment in which the logging tools have to work in the boreholes, by exposing it in particular to very high temperatures. It has the disadvantage of a certain fragility, an insufficient energy reserve for long-term logging operations. and reliability questionable. In the absence of this solution, it is advisable to surround the conductors which supply the oscillator from the tool suspension cable with a screen produced by a longitudinal metal tube passing through the probe to the oscillator 60. In in the case of coil tools where the power emitted or received depends on the flux passing through the coils, the passage of this tube is at the expense of the surface available for the flux. On the contrary, the operation of the two-plate antennas is not influenced by the presence of this tube, the diameter of which can be widened to accommodate the electronics itself and the other functional organs of the tool. This can shorten the tool or increase the electronic processing capacity inside it.

It is possible to plan to use the cylindrical mass elements such as 126 (FIG. 6) of the antennas as screens with regard to the disturbing signals circulating in the power cables of the oscillator and in any other electrical circuit necessary for the operation of the tool, connecting them to each other by metal connecting tubes so as to form a continuous cylindrical sheath. Thus, the screen tube formed by this sheath is integrated into the antenna sleeves placed a short distance from the periphery of the envelope of the mandrel 40 and releases the internal central part of the tool.

In some cases, however, it could be feared that this metallic screen sheath would play, in combination with the column of mud surrounding the mandrel 40 and the insulating media which separate them, the role of a coaxial connection in which part of the electromagnetic energy emitted at the output of transmitter 50 would propagate in a propagation mode transverse, known as TEM mode (TRANSVERSE ELEC-TROMAGNETIC MODE). Such a mode of propagation is to be avoided in the context of the use of electromagnetic logging tools. Indeed, the wave which is thus propagated between the emitter and the receivers along the coaxial thus formed between the sheath internal to the tool and the mud has an amplitude much greater than that of the signals which it is desired to capture and may completely obscure these with respect to the detection electronics. It is therefore important to seek to avoid this mode of propagation and to concentrate as much as possible of the energy leaving the emitter 50 (T) towards the wall of the hole and the surrounding formatiors.

According to one embodiment, provision is made to limit transmission in TEM mode by placing the conductive element wound around the sleeve of the antenna coaxial with the mandrel, not flat as in the case of FIG. 6, but on the wafer, as illustrated in FIG. 7 which shows an antenna 150 comprising a dielectric sleeve 152 comprising, on its internal face, a cylindrical ground conductor 154 connected to a cylindrical sheath, not shown, which traverses the entire height of the tool and connects in particular all the corresponding ground conductors of the other antennas. Around this mass cylinder 154 is wound, electrical contact with this cylinder, on its. wafer 155, a metal strip 156 which is placed on the field in a plane approximately perpendicular to the surface of the cylinder 154 and wound in a helix around the latter over a length of sleeve 152 corresponding to the length of the antenna. The outer edge 157 of the strip 156 is flush with the outer surface 160 of the dielectric sleeve 152. A conductive metal strip 158 forming the first element of the antenna is helically wound in the thickness of the sleeve 152 by forming turns arranged on the field and interposed between the turns of the ground element 156 and whose outer edge 159 is flush with the outer surface 160 of the dielectric sleeve 152. The strip 158 is electrically connected at one of its ends 162 to the metal coating 154 internally sheathing the sleeve dielectric 152. The antenna thus formed is connected to a coaxial cable 164, one end of which is perpendicular to the internal surface of the sleeve 152. The sheath of this cable is electrically connected to the ground cylinder 154 and the core of this conductor is connected to the strip 158 ′ at a point 166 at a suitable distance from the end 162 along this winding, to carry out the impedance matching of which it has been spoken. The helical winding of the strip 158 extends over a distance along the cylinder 152 corresponding to the developed length of the desired antenna.

This arrangement makes it possible to reduce the transmissions of electromagnetic energy in a direction parallel to the axis of the mandrel 44. In fact, the formation of waves capable of propagating in TEM mode inside the coaxial link defined above is mainly based on the existence of a capacitance distributed between the turns of the first wound antenna element 158 and the column of mud surrounding the tool. The value of this capacity depends on the effective surfaces of the facing reinforcements. In the case of FIG. 7, the surface of the edge of the turns 158 which are located opposite the column of mud is very small and therefore contributes minimally to the formation of waves transmitted according to the TEM mode.

A metal sleeve 170 is placed inside the insulating envelope 171 of an electromagnetic logging tool mandrel (FIG. 8), in the extension of the cylindrical mass element 172 of an antenna 173 of the biplate type. with field conductors 176 and 178, similar to the antenna shown in FIG. 7. The metal conductor on wafer 178 is embedded in the dielectric 175 and is electrically connected by its internal wafer to the ground cylinder 172. Power is supplied as before by a coaxial cable 179.

Finally, in all cases where it is desired to combat the transverse propagation mode (TEM) for the reasons which have just been indicated, the first element of the antenna is preferably wound in a helix (FIG. 9 ) form of two longitudinal portions wound in equal steps and in opposite directions. Thus, on a dielectric sleeve 192 whose inner part is sheathed with a metallic coating 193 forming a cylindrical mass element, an external metallic blade 194 is wound in a helix, starting from one end 195 short-circuited with the cylinder of mass 193, in one direction to a turning point 196 corresponding to half the developed length of the antenna, from which the pitch of the winding reverses to form the turns 197. The end 198 of this radiating element is short-circuited with the ground plane. The total length of the antenna is tuned to half the wavelength envisaged in the dielectric 192. It is supplied at two points close to its ends 195 and 198 respectively by two coaxial cables under two voltages in phase opposition. We therefore obtain two quarter wave antennas in series, geometrically opposite, but in which the current is in the same direction, which favors radiation in desired mode, while the electric fields corresponding to the voltage between each of the winding portions. reverse steps and the column of mud are of different signs and have opposite effects which tend to cancel the transmission in TEM mode.

According to another embodiment, a probe 200 (FIG. 10) comprises a cylindrical mandrel surrounded by an external metal casing 202. Around this envelope are arranged, in several longitudinally spaced locations 204, two-plate antennas 206 of the winding type according to the embodiment of FIG. 6. As shown in the upper part of FIG. 10 which shows the probe partially cut by a longitudinal diametral plane, the metallic outer casing 202 has, at each location 204, a portion 208 of narrowed outer diameter which in itself constitutes a cylindrical element of mass for each antenna 206. Each of these narrowed portions 208 is coated with a dielectric sleeve 209 around which a helical metal blade 210 is wound, one end of which is short-circuited with the narrowed metal part 208 of the conductive envelope 202. The combined thickness of the dielectric 209 and of the winding 210 is such that the diameter of the assembly is less than the diameter of the envelope 202 in the parts 212 which separate the locations 204. These portions 212 are interconnected by a series of bars 214 of longitudinal direction passing over each winding 210 which thus form a cage with parallel bars around the antennas 206 for the purpose of mechanical protection. The longitudinal bars 214 are integral with the casing 202. In this embodiment, the cylindrical mass element of the antennas is in direct electrical contact with the mud 202. No propagation in TEM mode can take place in the absence of a structure of the coaxial type with dielectric between an internal conductor and the column of mud surrounding the tool.

According to another embodiment (FIG. 11a), the envelope of a probe 220 is constituted by a cylindrical metal tube 222 extending over the entire height of the tool. Around the casing 222 are mounted, in longitudinally spaced positions, a transmitting antenna 224 at the bottom of the tool and a series of receiving antennas 226 ₁ , 226 ₂ , 226 ₃ and 226 _{4 "} Each of the antennas 224 , 226 ₁ to 226 ₄ comprises a dielectric coating 229 directly attached around the external surface of the envelope 222, which forms a cylindrical element of mass common to all these antennas. Around each dielectric sleeve 229 (FIG. 11b) is wound in helix a radiant metal strip 230 electrically connected to the casing 222 at one of its ends 231. The winding 230 is embedded in an insulating coating 232 based on fiberglass which gives it both mechanical protection against shocks and abrasion due to movement of the tool inside the borehole and chemical against corrosion.

A coaxial cable 234 for supplying the antenna 224 through the casing 222. Its sheath is electrically connected lie to this envelope. The core 235 is connected as previously described to carry out the impedance matching.

If the protection offered by the coating 232 is effective from a mechanical and chemical point of view, it is not essential from an electrical point of view. No propagation in TEM mode is to be feared.

The internal electronics necessary for the operation of the tools is housed (cut parts of FIGS. 10 and 11a) in the internal space delimited by the conductive tubes 202 and 222 at their upper part. Supports 240 mount electronic processing cards suitable for being connected to pairs of receivers 226 ₁ , 226 ₂ and 226 ₃ , 226 ₄ via coaxial cables 241, 243, 245 and 246.

The constitution of the electronic circuits and of the appropriate links to supply the transmitter 224 and to allow the processing of the signals coming from the receivers 2261 to 226 ₄ , is well known and, for example, described in the aforementioned United States Patent 4,185,238 of January 22, 1980 delivered to Messrs HUCHITAL and TABANOU.

Thus, an electromagnetic wave propagation logging probe has been produced comprising a metal envelope which, in addition to suppressing the propagation of waves in TEM mode, has a certain number of advantages for the production of tools, both on the in terms of robustness and ease of assembly and therefore of manufacturing cost. They can in particular make it possible to minimize or take into account variations in the spacing of the coils along the mandrels under the effect of thermal expansion.

The embodiments which have just been previously reviewed for mounting antennas on the mandrel of electromagnetic logging tools have a certain number of advantages over the embodiments of the prior art, in particular over tools using to coils as electromagnetic energy transducers between the tool and the surrounding environment.

An essential advantage compared to these previous embodiments of transducers lies in the improvement of the performance of these antennas. It has been found that the efficiency obtained is several tens of times and even up to almost a hundred times the efficiency of the coils used previously.

This improvement in efficiency is available, not only on the energy transmitter but on the receivers. With antennas whose efficiency is multiplied, for example by 50, the electric power corresponding to the signals received for the same power of transmission of electric signals is 2500 times higher. The useful powers which can be brought into play for the study of propagation are therefore of an order of magnitude completely different from what was possible up to now.

Many consequences follow from this improvement. Of course, it can be used to simplify the detection electronics associated with the receiving antennas R1 to R4 (Figure 1). This very significant increase in yield is obtained with antennas which have a space requirement of the same order of magnitude as that of the antennas of the prior art and which accommodate a conventional mandrel diameter for logging tools. Unlike transducers formed from coils whose efficiency increases with their diameter, the efficiency of the antennas according to the invention and therefore the amount of radiant energy capable of passing through these antennas is independent of their diameter. This considerable increase in the efficiency of the antennas is accompanied by an increase in the space available to house the electronics, even when supplying a transmitter at the base of the tool with power conductors in from the surface. It is therefore possible to reduce the total length of the tool. In general, the high level of performance of these years tennes makes it easy to envisage losing a few yield points as part of a compromise aimed at improving the characteristics of the tool.

The increase in radiated power makes it possible to apply the method of investigation of formations by electromagnetic propagation with drilling muds much more conductive than in the past. Indeed, the attenuation of the waves propagated in the medium increases with the conductivity as one exposed previously. With this new tool, we can accommodate a rate of attenuation of the energy radiated in the mud surrounding the drilling much stronger than what was acceptable until now, without reducing the total amount of energy likely to be detected at the receptors themselves.

Thus, it is possible to operate in sludges whose conductivity can reach 0.05 ohm, which makes it possible to extend the application of this type of tool to almost all of the fbrage sludge currently used. Similarly, it is no longer essential to try to increase the diameter of the tool in order to minimize the thickness of the column of mud crossed by the radiation on emission and on reception.

Another consequence of the considerable increase in radiated power lies in the increase in the signal-to-noise ratio at the receivers, an increase which makes it possible to improve the accuracy of the detection carried out while using simpler and possibly more reliable electronics to obtain Finally, we have seen that with this type of antenna, one could advantageously use a conductive tube over the entire length of the probe and which functions as a screen vis-à-vis the sensitive elements of these antennas, without generate unwanted propagation in TEM mode, through various means.

A probe fitted with antennas in accordance with the invention also makes it possible to increase the resolution of the investigation while maintaining a depth of investigation sought. The latter depends on the distance between the transmitters and the receiver, while the resolution depends on the distance between the receivers of each pair. The measurements made by each pair of receivers are differential measurements of the variation of certain propagation parameters such as attenuation or phase shift caused by a, ₇ formation zone whose thickness is defined by the spacing of these receivers ( see in particular the United States Patent 4 185 238 cited above). The smaller this spacing between the receivers, the smaller the thickness of this area and, consequently, the better the resolution of the measurement carried out. However, it is necessary to maintain the spacing of the receivers of a pair at a sufficient value so that the difference between the values of the parameters of the received waves can be perceived and measured by the detection and processing electronics. Thus, for example, if one wishes to detect the differences in phase shift to within a fraction of a degree, the spacing of the two corresponding receivers must be sufficient for the variations of this parameter between two formations of a different nature to distinguish have at least this value on the formation thickness considered. The same goes for mitigation.

To increase the resolution, we are therefore led to seek an increase in the measured attenuation and phase shift values of the electromagnetic waves propagating in a formation zone of given thickness, which can be obtained by increasing the operating frequency of the tool. However, because of the greater attenuation of the signals which accompany this increase in frequency, the radiated power must be appreciably greater if one wishes to be able to collect, at the level of the receivers, powers still sufficient to be detected there. and treated properly. This is possible with the tools equipped with antennas which have just been described, thanks to the considerable increase in the yield which can be obtained therefrom. So, with a power to the emission oscillator of a conventional order of magnitude, it is possible to radiate powers high enough to subject them to a much higher attenuation, in particular by operating at relatively higher frequencies compared to l prior art, while maintaining an acceptable signal level at the level of the receivers for detection and actual measurement. This result is achieved, while retaining a significant depth of investigation which involves a relatively long path of the electromagnetic waves between the transmitter and the receivers, during which the attenuation of a signal of relatively higher frequency can be considerable. .

The power emitted with antennas of the type described is sufficient to reach investigation depths of the order of one to two meters with antennas mounted on a mandrel in a frequency range which can rise up to about 60 to 80 megahertz. The resolution that can be obtained for this operating frequency is then from 60 centimeters to one meter. As soon as the operating frequency exceeds a few tens of megahertz, it is possible to make windings on mandrels of a useful length equal to half a wavelength and beyond to a wavelength, insofar as the wavelength decreasing when the frequency increases, it is possible, for a given longitudinal size of the antenna along the mandrel, to tune the developed length of the conductive element wound over a larger fraction of the length d wave of radiation.

In the relatively low frequency range, the developed length of antenna necessary to obtain agreement with the quarter wave of propagation can become prohibitive. However, the improvement, all other things being equal, of the intrinsic efficiency of these antennas, is sufficient to make their application attractive at frequencies very much less than 20 megahertz and which can drop below 1 megahertz. At these frequencies, a means for dimi to alter the useful length of the radiant element wound around the mandrel (first conductive element) consists in modifying the connections between this element and the earth element of the antenna.

A radiant winding of a two-plate antenna 255, formed of a metal strip 250 shown in FIG. 12, has a developed length less than a quarter of the wavelength. Instead of being in short circuit with the ground plane 254, the end 252 of the element 250 is connected to the ground plane 254 by an inductor 258 dimensioned so that the resonant assembly formed by the radiant element 250 and the inductor 258 coupled in parallel by the linear capacitance between the two conducting elements of the antenna 250 and 254 constitute a circuit tuned for the frequency considered. The injection point 256 is connected to an oscillator 257. The other end 253 of the element 250 is electrically free.

According to an alternative embodiment (FIG. 13) of an antenna on a mandrel 261 intended to operate at relatively low frequency, a radiant element 260 of developed length less than a quarter of the radiation wavelength in the dielectric 263 at the frequency envisaged has one of its ends 262 connected in short circuit to a ground plane. Its other end 265 is connected on the contrary to the ground plane 264 by a capacitor 266 whose value is chosen so that the circuit equivalent to the radiant element 260 and to the capacitor 266 coupled to the linear capacitance existing between the element 260 and the ground plane 264 constitutes a resonant circuit for the operating frequency of the oscillator 257.

Another means of reducing the useful length of the stiffening element wound around the mandrel, which can be combined with the previous one, consists in using, instead of a dielectric material between the two conducting elements of the antenna, a material to be high magnetic permeability. Indeed, while the dielectric constants with respect to air of common materials vary in proportions which hardly exceed 1 to 20, in general, the magnetic permeability of certain materials with respect to that of air can reach several thousand when the frequency is not too high, that is to say remains less than a few tens of megahertz.

If we refer to the relation (V), we see that the wavelength varies inversely proportional to the square root of the magnetic permeability. Under these conditions, it is possible for antennas operating in a frequency domain of less than a few tens of megahertz to select a magnetic material whose Curie point is sufficiently high so that it retains its high value of magnetic permeability throughout the domain. operating temperatures of drilling logging tools.

Thus, if a material such as ferromagnetic alumina is used, the magnetic permeability of which is around 250, an antenna can be formed with a first element plated on the external face of a sleeve of this material, the internal face is coated with a ground plane. If such an antenna is given a developed length of 38 meters, corresponding to a quarter of the wavelength in this material (as opposed to 600 m in vacuum), it is possible to operate this antenna at a frequency of 500 kHz. Such an antenna length can be reached using a winding of 120 turns on a 10 cm diameter mandrel.

Thus, a logging tool is produced by propagation of electromagnetic radiation capable of operating at a relatively low frequency, less than 1 MHz, a frequency in which the measurements of the parameters relating to the propagation of the electromagnetic waves in the formation are essentially influenced by the conductivity. of the surrounding media and relatively little by the dielectric constant of these media. By mounting the antennas on a metal mandrel, as explained above, we obtain a new conductivity tool which, unlike conventional conductivity tools, does not require the use of an insulating mandrel on at least a large part of its length. This new type of tool offers advantages of simplicity, robustness and dimensional stability.

This new structure also makes it possible to combine a conductivity measurement tool with an electrode resistivity measurement tool by associating antennas operating at relatively low frequency of the type which has just been indicated with metal sleeves arranged around the mandrel and constituting electrodes, for example, of a tool of the Double Laterolog type. described in United States Patent 2,712,627 filed by H.G. DOLL.

A new combined tool of this type comprises (FIG. 18) a probe 400 on which is mounted a central electrode A _o . On either side of A _O are symmetrically arranged two pairs of closely spaced potential electrodes M ₁ , M2 and M ' ₁ . M ' ₂ followed by moving away from the electrode A _O of first current electrodes A ₁ and A' ₁ then of second current electrodes A ₂ , A ' ₂ . The electrodes consist of conductive rings on the surface of the mandrel, the electrodes A ₂ and A ' ₂ taking the form of elongated sleeves. The supply and control circuits of these electrodes are well known, for example from United States Patent 3,772,589 filed by A. SCHOLBERG. They make it possible to pass currents between the electrodes A _o , A ₁ and ^A ₂ and A _O , A ' ₁ and A' ₂ and zones at greater or less distance from the tool in the surrounding medium by controlling these currents in such a way that the potential difference between the measurement electrodes M ₁ and M ₂ and between the electrodes M ' _l and M' ₂ remains zero.

As a reminder, on the left side of the tool, the line lines 412 have been drawn in the shallow investigation configuration and, on the right side, the line of these lines in the deep investigation configuration where the current lines return to the borehole at a point distant from the set of electrodes on the tool, for example a ground electrode located on the cable 410 for supporting the probe 400 The currents transmitted by the electrodes A ₁ , A ₂ , A ^s ₁ and A ′ ₂ are controlled so as to force the current transmitted by the electrode A more or less far in the formation according to the desired depth of investigation.

A two-plate antenna 401 of the type described above, for example with reference to FIG. 6, is mounted at one end of this set of electrodes, below the electrode A ′ ₂ . It is connected to an oscillator operating at a frequency of 500 kilo hertz to transmit radiation of corresponding frequency to the training.

Two receiving two-plate antennas 404 and 406 are mounted, the first between the electrodes A ' _l and A' ₂ and the second between the electrodes A ₁ and A ₂ . They are connected to electronic acquisition and processing of signals picked up after propagation in the formation to provide a differential measurement of conductivity. The combination of the measurements carried out by the receivers 404 and 406 makes it possible to obtain such a differential measurement of conductivity which is corrected for the influence of the zone immediately in the vicinity of the tool, that is to say of the hole itself. and the surface area around the wall of the hole.

The antennas 401, 404 and 406 have a common ground plane conducting at the frequency of 500 kilo hertz. The current electrodes A _o , A ₁ , A ' ₁ , A ₂ , A' ₂ are all electrically connected to this ground plane. Thus, the ground plane is in electrical contact with the mud at various points along its length. However, the ground plane is arranged using appropriate capacitive links between its different portions so that at the operating frequency of the Double Laterolog which is a few hundred hertz these electrodes are not electrically connected by the plane massive.

The radiating elements of the antennas 401, 404 and 406 are of the type of the element 124, in FIG. 6 for example. They are plated on a sleeve such as 120 of ferromagnetic alumina, the magnetic permeability of which is approximately 250.

Such a tool can advantageously replace an existing tool performing the same combination of functions in which the current inducing elements in the formation are coils. In order to avoid the induction of currents in annular electrodes which would falsify the measurements carried out using induction coils, each electrode of this tool is produced in the form of a series of buttons surrounding the mandrel, isolated from each other on the surface of it.

The presence of two-plate antennas avoids having to resort to button electrodes and allows the use of massive electrodes whose performance in depth of investigation is better. In addition, the performance of the conductivity meter with two-plate antennas of FIG. 18 is improved compared to the coil tool in terms of better vertical resolution and less influence of the immediate environment. hole due to the differential nature of the measurement and higher signal levels.

The antennas for electromagnetic logging of the type described are also usable at the other end of the frequency spectrum envisaged, in particular at frequencies above 200 MHz. Independently of their use on mandrels of tools plunged in drilling muds, these antennas, by their very simple realization, can also be applied to skids to operate at high frequencies. When the frequency increases, it is often preferred to use antennas mounted on pads having a longitudinal dimension compatible with a relatively small depth of investigation.

According to one embodiment, such antennas can be produced in the form of buttons (FIG. 14) in which a patch of dielectric material 300 is coated on one of its faces with a conductive metallic layer 302 forming a ground plane and comprises a thin metal strip 306, printed on its opposite face 304 in the form of a spiral between a central end 308 which can be electrically short-circuited with the ground plane 302 and another end 310 which is left free if the developed length of the printed spiral is equal to a quarter of the wavelength of propagation of the radiation in the dielectric 300 or to an odd multiple thereof and which is short-circuited with the ground plane 302, as in the example shown, if the developed length is equal to or a multiple of the propagation half-wavelength.

Such buttons can be integrated into an elongated pad 321 of the type shown in FIG. 16 which has at each end of its face for pressing against the wall of a borehole, a transmitter button similar to that shown in FIG. 14, respectively 320 and 322 connected to a very high frequency power oscillator not shown. Between the two transmitters 320 and 322 are mounted, in the vicinity of the central part of the shoe, three receivers, respectively 322, 324 and 325 aligned in the longitudinal direction of this shoe. The shoe 321 is mounted (FIG. 1) in a known manner on the probe mandrel 44 and articulated so as to be able to be applied by arms 43 against the wall 28 of the borehole, either under the action of a permanent elastic stress exerted by springs in the shape of an arc, either by means of arms whose opening is controlled remotely.

An arrangement with two transmitters 320 and 322, in accordance with that of FIG. 16, is used when it is sought to compensate for the effects of a defect in uniform application of the shoe against the wall of the hole under the effect of the irregularities of this last. Techniques for compensating for the influence of the borehole (technique known as compensation BHC) are well known for tools operating with different types of transducers, for example by United States Patent No. 3,257,639 filed by KOKESH.

A shoe 321 such as that of FIG. 16 operating in a frequency range from 60 MHz to 3 gigahertz (ultra-high frequencies) can be applied to a pendulum measurement tool. Measurements made from detectors 323 to 325 make it possible to detect with high resolution variations in the dielectric characteristics of the formation strata traversed by the drilling. According to known pendulum measurement techniques, all of the indications from three and preferably four similar pads, as described by United States Patent No. 3,423,671 filed by A. VEZIN, applied around the wall of the hole , makes it possible to determine the inclination of the planes of separation of the contiguous strata. Two-plate antennas of the type described above advantageously make it possible to adapt this tool to boreholes filled with unsalted water or petroleum-based drilling muds in which conventional tools with electrodes are inoperative. In addition, such a tool (FIG. 1) equipped with such antennas 321 makes it possible to obtain, in each position of the pad, both a measurement of conductivity and a measurement of dielectric constant which are taken up to correlate these measurements between and improve the use of all the data collected. We thus obtain extremely fine representations of all the variations in geological characteristics, fractures and other discontinuities of the formations crossed by the drilling.

The invention can also be applied to the production of pads for transmitting and receiving electromagnetic energy at an extremely high frequency of the type shown in FIG. 17 in which a pad 370 is provided with a transmitting antenna 372, towards its lower end, produced in two-plate technique and comprising in particular a metal ring 373 printed on a dielectric forming the surface of the pad 370, electrically connected, at a point 371, to a ground plane placed on the other side of the dielectric surface, and whose perimeter is suitable for constituting a half-wave antenna at a high operating frequency which can be, for example, 850 MHz. Two receivers, towards the other end of the pad, are formed by two bars 374 and 375, aligned on the surface of the pad in the longitudinal direction of the latter and of the antenna 372. These bars 374 and 375 are electrically connected to one of their ends 376 and 377 to the ground plane located on the other side of the dielectric forming the surface of the pad and their length corresponding to a quarter wavelength for the frequency envisaged. This pad makes it possible to detect certain parameters of the propagation of the electromagnetic energy emitted by the transmitter 372 near the wall of the borehole and in particular in the mud cake (mudcake) if there is one.

The antennas according to the invention can also be applied to investigations in the very high frequency (VHF) scale, ie frequencies between 200 and 500 MHz.

For this purpose, a pad 380 (FIG. 15) for close investigation (pad R) is constituted by an insulating plate 380 made of an abrasion-resistant material, such as a composite of fiberglass and resin, elongated in a direction corresponding to a longitudinal movement against the wall of a borehole. This shoe comprises a lower transmitting antenna 384, surmounted in the longitudinal direction by two receiving antennas 386 and 388. Its rear face, that is to say not facing the formation, has a continuous metallic coating forming a ground plane, embedded in a protective sheath with respect to drilling fluids in which the pad is suitable for bathing. Each of these antennas comprises, in addition to the ground plane and the dielectric layer forming the thickness of the pad itself, a metal blade 3R5, 386 and 388 respectively, printed in the anterior surface 382 of this pad according to a spiral path composed of straight segments joined at sharp angles to cover a substantial portion of the surface of the skate. The antenna 384 occupies the lower part thereof, the printed metal strip 385 of this antenna being short-circuited with the ground plane at its central end 391 and at its peripheral end 392, its length being determined to constitute an antenna half wave. The two receiving antennas each have a central end, respectively 393 and 394, short-circuited with the ground plane on the rear face of the pad, tan disk the other end of the metal strip of these antennas is left free, the length of these blades being equal to a quarter of the wavelength of the envisaged radiation.

Such a type of shoe is of particular interest in sludge based on unsalted water or petroleum where the electrode shoes do not work. It also has the particularity of having a lower sensitivity to the resistivity of the mud, the influence of which it eliminates by differential measurement and makes it possible to obtain a better knowledge of the resistivity of the water of formation Rw. Indeed, the measurement of the dielectric constant near the wall of the borehole makes it possible to know the water saturation Sw in the area called the invaded area where, under the effect of the pressure of the drilling mud in contact with the wall of the hole, the filtrate of the mud has penetrated after having deposited the solid particles of the latter on the wall of the hole (to form the mud cake) by displacing a part of the hydrocarbons likely to be in the pores of this formation.

The metal parts of the antenna on pads shown in Figures 14 to 17 are advantageously coated with an insulating protective coating to better resist both mechanical abrasion and corrosion.

Claims (10)

₁ . Logging tool for measuring propagation characteristics of an electromagnetic wave in the environment surrounding a borehole crossing geological formations, comprising a probe (40) suitable for being moved in this borehole, means on this probe suitable for carrying out a conversion between electrical signals in the probe and electromagnetic energy signals propagating in the environment surrounding this probe, characterized in that said means comprise at least one antenna comprising a first elongated element (102) conducting electricity and a second conductive element (105) disposed opposite and at a distance from the first element (102) over the entire useful length of the latter, the opposite portions of the first and second elements being separated by an electrically non-conductive medium ( 100) and electrically connected to each other at one end (106) of the useful length of the first element. ₂ . Tool according to claim 1, characterized in that the electrical connection between the first and the second element is a short circuit (108) and the useful length of the first element is granted as a function of the propagation wavelength of said energy electromagnetic frequency determined in the nonconductive medium. 3. Tool according to one of the preceding claims wherein said antenna is adapted to be connected to a link (109) for the transmission of electrical energy at a determined frequency between this antenna and an electrical circuit in the probe, characterized in that that the position of the point (110) of connection of this antenna to said link is chosen along said first conductive element so as to adapt the impedance of this antenna to said link. 4. Tool according to one of the preceding claims, characterized in that the first conductive element is an element (124) elongated along a curvilinear line parallel to a surface and the second conductive element (126) is a two-dimensional element parallel to this surface. 5. Tool according to one of the preceding claims, characterized in that the first conductive element (124) is placed relative to the second conductive element (126) on the outside of the probe. 6. Logging method for measuring the characteristics of the environment surrounding a borehole passing through geological formations, comprising the steps of moving a probe (40) in this borehole, carrying out by means of this probe a conversion between signals electrical signals in the probe and electromagnetic energy signals propagating in the surrounding medium, characterized in that said conversion is carried out by means which comprise at least one antenna comprising a first elongated conductive element (102) (105) arranged opposite and at a distance from the first element (102) over the entire useful length of the latter, the facing portions of the first and of the second element being separated by an electrically non-conductive medium (100) and electrically connected to each other other at one end (106) of the useful length of the first element. 7. Method according to claim 6, characterized in that said conversion is carried out by granting the useful length of the first element as a function of the propagation wavelength of said electromagnetic energy of frequency determined in the non-conductive medium. 8. Method according to claim 6, characterized in that said conversion is carried out so that the end of the first conductive element is electrically connected to the second conductive element by means of a self-inductance. 9. Method according to one of claims 6 to 8, characterized in that said conversion is carried out with the first element. conductive ment (124) elongated along a curvilinear path parallel to a surface and the second conductive element (126) parallel to this surface. ₁₀ . Method according to one of claims 6 to 9, characterize _s ed in that said conversion is performed with the first conductive element (124) positioned relative to the second conductive element (126) on the outside of the probe.

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